Silicone rubber compositions are used in a wide variety of applications because they form cured products featuring the favorable properties of weathering and heat resistance and excellent rubbery properties such as hardness and elongation. Because of their surface tack, however, dust deposition is a problem when they are used as coatings on electric and electronic parts. See JP-A 2001-002922.
A silicone varnish which has solved the problem suffers from a crack problem. It would be desirable to have a silicone rubber composition forming a cured product which eliminates dust deposition on its surface when used in the packaging of electric and electronic parts and which has good crack resistance.
In general, it is known in the art that a cured silicone rubber composition of the addition cure type can be improved in strength by compounding a resinous organopolysiloxane. Although the compounding of a resinous organopolysiloxane is effective for enhancing the strength of a cured product, the product still maintains surface tack, posing the problem of dust deposition.
Meanwhile traditional light-emitting semiconductor devices such as light-emitting diodes (LED) are lamp-type light-emitting semiconductor devices in which a light-emitting semiconductor member is disposed on a lead electrode and encapsulated with a transparent resin to a cannonball shape as shown in FIG. 3. They are recently replaced by the “surface mount” type as a result of simplification of the mounting technology. Nowadays surface mounting light-emitting semiconductor devices as shown in FIGS. 1 and 2 become the mainstream.
In FIGS. 1 to 3, the device includes a housing 1 of glass fiber-reinforced epoxy resin, a light-emitting semiconductor member 2, lead electrodes 3 and 4, a die-bonding material 5, gold wires 6, and an embedding/protecting material 7.
While resin compositions are used for the embedment of light-emitting semiconductor members such as LED, it is required that the cured resin compositions be transparent. Then compositions comprising an epoxy resin such as a bisphenol A epoxy resin or alicyclic epoxy resin and an acid anhydride curing agent are generally used (see Japanese Patent No. 3,241,338 corresponding to JP-A 11-274571 and JP-A 7-025987).
However, these transparent epoxy resins have drawbacks including poor durability to moisture due to a high percent water absorption, poor durability to light due to a low transmittance of short wavelength light, and coloring due to photo-degradation.
Under the circumstances, resin compositions comprising an organic compound having at least two carbon-to-carbon double bonds (which are reactive with SiH groups) in a molecule, a silicon compound having at least two SiH groups in a molecule, and a hydrosilylating catalyst were proposed for the embedment and protection of optical semiconductor members (see JP-A 2002-327126 and JP-A 2002-338833).
Regrettably, such silicone compositions have a drawback that when an attempt is made to improve the crack resistance, the cured composition retains surface tack so that dust readily deposits on the surface to interfere with light transmission. It was then proposed to use high-hardness silicone resins for the embedment and protection purposes (see JP-A 2002-314139 corresponding to US 2002-0145152 and JP-A 2002-314143 corresponding to US 2002-0190262).
The high-hardness silicone resins, however, are less adhesive. In an encased light-emitting semiconductor device comprising a light-emitting member disposed in a ceramic and/or plastic housing, wherein the housing interior is filled with a silicone resin, a problem arises in a thermal shock test between −40° C. and 120° C., that the silicone resin separates from the ceramic or plastic housing.
Another problem arises from the fact that optical crystals of various compound semiconductors used in light-emitting members, such as SiC, GaAs, GaP, GaAsP, GaAlAs, InAlGaP, InGaN, and GaN, have high refractive indices. If the refractive index of embedding/protecting resin is low as in the case of dimethylsilicone resin, light is reflected at the interface between the embedding resin and the optical crystal, resulting in a lower emission efficiency.
It is then proposed to add an antireflection film as a means of enhancing the outcoupling efficiency (see JP-A 2001-246236 and JP-A 2001-217467 corresponding to U.S. Pat. No. 6,614,172). The provision of an antireflection film undesirably adds preparation steps and increases the cost.